1. Field of the Invention
The present invention relates to an image forming apparatus, a control method for the same, and a storage medium for storing a program therein.
2. Description of the Related Art
Image forming apparatuses such as electrophotographic printers, copiers, facsimiles, and multifunction peripherals that serve as a combination thereof typically perform printing by forming a toner image on a printing medium based on image data, conveying the printing medium on which the toner image has been formed to a fixing device, and fixing the toner image.
The fixing device includes a heater for fusing toner, a fixing film for increasing the removability of the printing medium from the heater, and a pressure roller for pressing the printing medium against the fixing film while conveying the printing medium. The fixing device thus fuses the toner image on the printing medium and fixes the toner image to the printing medium within the fixing device by applying heat from the heater to the printing medium via the fixing film and applying pressure to the printing medium with the pressure roller while the printing medium is conveyed by the fixing film and the pressure roller.
Here, the temperature of the heater required for fusing and fixing toner varies depending on the thickness of the toner image on the printing medium. Therefore, Japanese Patent Laid-Open No. 2008-15039, for example, discloses technology by which the temperature of the heater of the fixing device is controlled to be as low as possible within a range in which a fixing failure does not occur at fusing and fixing of the toner image.
However, according to the above-described technology, the temperature of the heater is controlled by setting the same temperature for the entire printing medium, even when the toner image has a predetermined thickness only within a partial area of the printing medium. The following describes this situation with reference to FIGS. 1A to 1D.
FIG. 1A shows a situation in which a printing medium 2104 is conveyed to the fixing device and is subjected to fixing. The printing medium 2104 is sandwiched between a pressure roller 2103 and a fixing film 2102 and is subjected to heat and pressure by a heater 2101 and the pressure roller 2103, and thus a toner image 2105 is fixed to the printing medium 2104. FIG. 1B shows a situation at a point in time that is after fixing to the first printing medium 2104 has completed and that is immediately before fixing to the subsequent printing medium 2107 is started. The following examines a case where a toner image is present on the entire surface of the printing medium 2104, i.e., a case where the toner coverage ratio is high. In this case, if the temperature of the heater 2101 is at the minimum temperature required for fixing the toner image, moisture in the printing medium 2104, which has not completely evaporated, finds a way to the pressure roller 2103 side and dampens the pressure roller 2103. Consequently, slippage due to moisture occurs between the pressure roller 2103 and the fixing film 2102 before the subsequent printing medium 2107 has been conveyed. While this slippage is occurring, the same position on the fixing film 2102 is heated by the heater 2101 for a longer period of time compared to when slippage does not occur (FIG. 1C). Repetition of this situation leads to the occurrence of temperature unevenness in the fixing film 2102, and such temperature unevenness results in the occurrence of gloss unevenness after the completion of fixing to the subsequent printing medium 2107 as indicated by a reference numeral 2108 shown in FIG. 1D.
Note that the toner coverage ratio is the ratio of the number of pixels on which any of cyan, magenta, yellow, and black is present relative to the total number of pixels in one page that has been subjected to image forming. Specifically, the toner coverage ratio can be obtained by counting the number of white pixels in which no cyan, magenta, yellow, or black is present from among the total number of pixels on a single page after an image has been formed on a printing medium, and then performing the following calculation:Toner coverage ratio=(Total number of pixels−Number of white pixels)/Total number of pixels